This invention relates to a motor-driven injection molding machine comprising an injection device which is driven by servomotors and, more particularly, to an improvement of a depressurization method in a heating cylinder in a plasticization and metering process.
In recent years, motor-driven injection molding machines have been widely used. The motor-driven injection molding machines uses, as actuators, servomotors in lieu of hydraulic actuators. Now, the description will proceed to operations of such an injection device using the servomotors as the actuators in brief.
(1) In the plasticization and metering process, a screw is rotated with a screw-rotation servomotor. The screw is located within the heating cylinder. Resin powder is fed, as fed resin, from a hopper to a rear portion of the screw in the heating cylinder. Rotation of the screw results in feeding the fed resin having a predetermined metered amount to a nose portion of the heating cylinder with the fed resin molten by the heating cylinder to form molten resin. During this time duration, the screw is retracted due to a pressure (back pressure) of the molten resin which is trapped in the nose portion of the heating cylinder.
The screw has a rear end portion which is directly connected to an injection shaft. The injection shaft is rotatably supported on a pressure plate through a bearing. The injection shaft is driven in an axial direction by an injection servomotor which is supported on the pressure plate. The pressure plate moves forward and backward along a guide bar in response to the operation of the injection servomotor through a ball screw. In the manner which will later become clear, the above-mentioned back pressure of the molten resin is detected by using a load cell and controlled with a feedback control loop.
(2) Subsequently, in a filling process, the pressure plate is advanced by means of driving the injection servomotor. The screw has a nose portion which serves as a piston to fill a mold with the molten resin.
(3) At the end of the filling process, the molten resin fills the while space within a cavity of the mold. At this point, the advancing motion of the screw has a control mode which is switched from a velocity control mode to a pressure control mode. This switching is referred to as a xe2x80x9cV-P switching.xe2x80x9d
(4) After the V-P switching, the resin within the cavity of the mold is allowed to cool under a predetermined pressure. This process is called a dwelling process. In the dwelling process, the resin has pressure which is controlled in a feedback control loop in the similar manner which is described in conjunction with the above-mentioned back pressure control.
Subsequently, operation of the injection device returns back to the plasticization and metering process set forth in (1) after the completion of the dwelling process set forth in (4).
On the other hand, in a clamping device, an eject operation for ejecting a solid product out of the mold is carried out in parallel with the plasticization and metering process set forth in (1). The eject operation involves in opening the mold to remove the solidified product from the mold by means of an ejector mechanism and thereafter in closing the mold for the resin filling set forth in (2).
At any rate, it is important for the plasticization and metering process that high accuracy is achieved. This may be theoretically achieved by stopping the rotation of the screw with the screw positioned at a predetermined constant stroke position. However, in practice, it is difficult to achieve this in the manner which will later be described in conjunction with FIGS. 3A through 3C and 4A through 4C.
It is therefore an object of the present invention to provide a depressurization method for a plasticization and metering process of a motor-driven injection molding machine, which is capable of controlling back pressure within a heating cylinder and a position of a screw with high accuracy on and immediately after the completion of the plasticization and metering process.
Other objects of this invention will become clear as the description proceeds.
One aspect of this invention is directed to a depressurization method in a plasticization and metering process which is for a motor-driven injection molding machine comprising an injection device. The injection device comprises a heating cylinder for heating resin powder therein to melt the resin powder into molten resin, a screw disposed in the heading cylinder for feeding the molten resin in the heating cylinder forward to meter the molten resin, a screw-rotation servomotor operatively coupled to the screw for rotating the screw, and an injection servomotor operatively coupled to the screw for driving the screw along an axial direction to inject the molten resin metered in the heating cylinder forward. The injection device is provided with a load cell for detecting back pressure of the molten resin in the heating cylinder that is metered ahead of the screw to produce a pressure detected signal indicative of the back pressure and with a position detector for detecting a position of the screw to produce a position detected signal indicative of the position of the screw. The motor-driven injection molding machine comprises a controller for controlling, in response to the pressure detected signal and the position detected signal, driving of the screw-rotation servomotor and of the injection servomotor.
According to a further aspect of this invention, the above-understood depressurization method in the controller method comprises the steps of positioning, in response to the position detected signal, the screw at a metering position using the injection servomotor on and immediately after completion of the plasticization and metering process, and of rotating, in response to the pressure detected signal, the screw in the opposite direction using the screw-rotation servomotor on and immediately after the completion of said plasticization and metering process to carry out depressurization of the molten resin in the heating cylinder that is metered ahead of the screw.
In the above-mentioned depressurization method, the controller preferably may rotate the screw in the opposite direction until the back pressure indicated by the pressure detected signal is lowered to a predetermined pressure. Desirably, the controller may determines, in accordance with a pressure difference between the back pressure and the predetermined pressure, a rotation speed of the screw-rotation servomotor on making the screw rotate in the opposite direction. In addition, the controller preferably may restrict the rotation speed of the screw in the opposite direction to the upper limit thereof. Furthermore, the controller desirably may restrict a time interval for which the screw rotates in the opposite direction to the upper limit thereof.
A further aspect of this invention is directed to a controller which is for use in a motor-driven injection molding machine comprising an injection device. The injection device comprises a heating cylinder for heating resin powder therein to melt the resin powder into molten resin, a screw disposed in the heating cylinder for feeding the molten resin in the heating cylinder forward to meter the molten resin, a screw-rotation servomotor operatively coupled to the screw for rotating said screw, and an injection servomotor operatively coupled to the screw for driving the screw along an axial direction to inject the molten resin metered in said heating cylinder forward. The injection device is provided with a load cell for detecting back pressure of the molten resin in the heating cylinder that is metered ahead of the screw to produce a pressure detected signal indicative of a pressure detected value of the back pressure and with a position detector for detecting a position of the screw to produce a position detected signal indicative of a screw position detected value of the position of the screw. Responsive to the pressure detected signal and the position detected signal, the controller controls driving of the screw-rotation servomotor and of the injection servomotor through first and second motor drivers by supplying the first and the second motor drivers with first and second actuating commands, respectively.
According to a further aspect of this invention, the afore-understood controller comprises a reverse rotating arrangement connected to the load cell for rotating, in response to the pressure detected signal, the screw in the opposite direction by supplying the first actuating command to the first motor driver on and immediately after completion of a plasticization and metering process to carry out depressurization of the molten resin in the heating cylinder that is metered ahead of the screw, and a positioning arrangement connected to the position detector for positioning, in response to the position detected signal, the screw at a metering position by supplying the second actuating command to the second motor driver on and immediately after the completion of the plasticization and metering process.